On Tue, Sep 22, 2015 at 04:23:33PM +0200, David Sterba wrote: > On Tue, Sep 22, 2015 at 01:41:31PM +0000, Hugo Mills wrote: > > On Tue, Sep 22, 2015 at 03:36:43PM +0200, Holger Hoffstätte wrote: > > > On 09/22/15 14:59, Jeff Mahoney wrote: > > > (snip) > > > > So if they way we want to prevent the loss of raid type info is by > > > > maintaining the last block group allocated with that raid type, fine, > > > > but that's a separate discussion. Personally, I think keeping 1GB > > > > > > At this point I'm much more surprised to learn that the RAID type can > > > apparently get "lost" in the first place, and is not persisted > > > separately. I mean..wat? > > > > It's always been like that, unfortunately. > > > > The code tries to use the RAID type that's already present to work > > out what the next allocation should be. If there aren't any chunks in > > the FS, the configuration is lost, because it's not stored anywhere > > else. It's one of the things that tripped me up badly when I was > > failing to rewrite the chunk allocator last year. > > Yeah, right now there's no persistent default for the allocator. I'm > still hoping that the object properties will magically solve that. There's no obvious place that filesystem-wide properties can be stored, though. There's a userspace tool to manipulate the few current FS-wide properties, but that's all special-cased to use the "historical" ioctls for those properties, with no generalisation of a property store, or even (IIRC) any external API for them. We're nominally using xattrs in the btrfs: namespace on directories and files, and presumably on the top directory of a subvolume for subvol-wide properties, but it's not clear where the FS-wide values should go: in the top directory of subvolid=5 would be confusing, because then you couldn't separate the properties for *that subvol* from the ones for the whole FS (say, the default replication policy, where you might want the top subvol to have different properties from everything else). Hugo. -- Hugo Mills | Putting U back in Honor, Valor, and Trth. hugo@... carfax.org.uk | http://carfax.org.uk/ | PGP: E2AB1DE4 |
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